This invention is generally in the field of switching techniques and relates to a method and device for all-optical switching.
Optical communication networks require cross-connect or switching mechanisms enabling direction, diversion, multiplexing or broadcasting (multicasting) of a plurality of information channels in a manner to meet the requirements of the network. Optical switches can also be used in Dense Wavelength Division Multiplexing (DWDM) telecommunication systems for routing the information, enabling optical add/drop multiplexing (OADM), as well as for protection purposes.
Switching modules have various forms, such as structures whereby N input channels are directed simultaneously to M output ports in various configurations. In a dynamic switch, this operation can be reshuffled in time.
The so-called xe2x80x9call-optical switchesxe2x80x9d are capable of performing the switching function without converting the signals from the optical domain to the electrical domain and back. Such an optical switch is disclosed, for example, in U.S. Pat. No. 6,041,151. This switch device utilizes a double refracting crystal operating as a polarizing beam displacer in conjunction with a controllable half wave gate that is able to rotate the polarization of an incident light beam by 90 degrees in accordance with a control input.
There is accordingly a need in the art to facilitate a switching technique by providing a novel all-optical switch device of a simple and compact design.
An all-optical switch device according to the invention does not depend on optics-electronics-optics (O-E-O) conversion, and does not need movement of the elements of the switch device. The switch device is characterized by fast operation exhibiting low insertion loss and minimal cross talk between output channels. The switch device is based on the electro-optic effect exhibited in a material of the kind capable of dynamically producing a phase delay for each polarization component of an incident light beam as a function of a voltage applied to the material. Such a material may be Lead Lanthanum Zirconate Titanate (PLZT), BSO or LiNbO3. The varying phase delay may result in a controllable polarization rotation.
The main idea of the present invention consists of designing a switch device having a polarizing beam splitting surface, controllable polarization rotating (CPR) means, and beam directing means. The CPR and/or beam directing means may be separate elements accommodated at opposite sides of the polarizing beam splitting surface. The polarizing beam splitting surface may be the surface of a beam splitter (e.g. cubic beam splitter). The CPR and beam directing means may be integral with the beam splitter, by making the entire beam splitter or respective portions thereof from a polarization rotating material and making respective surfaces of the beam splitter reflective. Alternatively, the CPR and beam directing means may be separate elements accommodated at respective surfaces of the beam splitter.
An input beam (either unpolarized or of a specific polarization) impinges onto the polarizing splitting surface, and can be split into two linearly polarized beam components (in the case of unpolarized input beam), which propagate along different optical paths. The beam components of the input beam interact with the CPR means and beam directing means, and return back to the polarizing beam splitting surface, where at least one output beam (e.g., of no particular polarization state, namely, consisting of different polarization components) is produced. It should be understood that, when a polarized input beam is used, the device of the present invention provides for directing this beam to a selected one of two output channels.
If the entire beam splitter is made of a polarization rotating material, then, in the operative state of the beam splitter, the different polarization components of an input beam undergo polarization rotation prior to being split by the polarizing beam splitting surface of the beam splitter. Depending on the current mode of the CPR means (which may and may not be integral with the beam splitter), namely, operative or inoperative modes of the CPR, the output beam can be directed towards one of the two output channels of the device, or two output beam components can be directed towards both output channels, respectively. The CPR in the inoperative and operative modes thereof does not affect and does affect, respectively, the polarization of the beam passing therethrough.
The controllable portion rotating medium is shiftable between its inactivated and activated states by application of an electric field to the medium. Depending on the type of the medium being used, one of its states presents an operative mode of the medium, and the other state presents an inoperative mode of the medium. Polarization rotating medium of the kind based on the electro-optic effect (e.g., ferroelectric crystals or ceramics) is in the operative mode (affecting the polarization of a beam), when in the activated state of the medium. The polarization rotating medium of the kind utilizing LC materials is in its inoperative mode (i.e., does not affect the polarization state of a beam), when in the activated state of the medium, and is in its operative mode (i.e., affects the polarization state of a beam), when in the inactivated state of the medium.
Hence, the terms xe2x80x9cinoperative modexe2x80x9d and xe2x80x9coperative modexe2x80x9d of a CPR medium are associated with effect of the medium with respect to an incident beam, irrespective of the terms xe2x80x9cactivatedxe2x80x9d and xe2x80x9cinactivatedxe2x80x9d states which are associated with the application of an electric field to the CPR. In the inoperative mode of the CPR, it does not affect the polarization of the beam, and in the operative state of the CPR, it affects the polarization of the beam.
Thus, according to one aspect of the present invention, there is provided a switching method for selectively directing an input beam to at least one of two output channels, the method comprising the steps of:
(i) providing incidence of the input beam onto a polarizing beam splitting surface to thereby enable splitting of the input beam into two beam components of different polarizations propagating along different optical paths;
(ii) passing the input beam components of different polarizations through a controllable polarization rotating medium capable of affecting the polarization of each of the beam components; and
(iii) directing the beam components that have passed through said medium onto said polarizing surface, thereby producing at least one output beam propagating towards at least one selected output channel, depending on a current mode of the polarization rotating medium.
It should be understood that the input beam may be unpolarized, namely a randomly polarized beam (of no specific polarization state), containing beam components of different polarizations. Alternatively, the input beam may have a specific polarization. In this case, the interaction of the input beam with the polarizing beam splitting surface will result in the beam propagation along a specific optical path.
It should also be understood that the input beam may pass the CPR medium prior to being split into the two beam components of different polarizations. This may be implemented by utilizing a beam splitter having the polarizing beam splitting surface and being made from a controllable polarization rotating material.
According to another aspect of the present invention, there is provided an all-optical switch device operable for selectively directing an input beam to at least one of two output channels, the device comprising:
(a) a polarizing beam splitting surface capable of splitting an input beam into two beam components of different polarizations and directing the split beam components to propagate along different optical paths, and capable of combining two beam components of different polarizations to produce at least one output beam;
(b) controllable polarization rotating means accommodated in optical paths of the input beam components, and selectively operable to affect the polarization thereof; and
(c) beam directing means accommodated in optical path of the beam components passed through the polarization rotating means for directing the beam components onto said polarizing beam splitting surface to thereby produce at least one output beam propagating towards at least one selected output channel.
Preferably, the polling beam splitting surface is a surface of a cubic beam splitter. The polarization rotating means may be in the form of two elements accommodated at opposite sides of the polarizing beam splitting surface.
For example, the polarization rotating means may be accommodated at surfaces of the beam splitter that intercept with the plane of the polarizing surface. In this case, the beam directing means may be in the form of two pairs of reflecting surfaces, each pair located at opposite sides of the corresponding one of the polarization rotating elements.
The two polarization rotating elements may be incorporated in two corner prisms, respectively, located at the adjacent surfaces of the beam splitter that intercept with the plane of the polarizing surface. Two corner prisms accommodated at said adjacent surfaces of the beam splitter and made from a polarization rotating material may be used, thereby function as both the polarization rotating and the beam directing means.
The polarizing beam splitter may be configured such that its two adjacent surfaces that intercept with the plane of the polarizing beam splitting surface are shaped like two-part right-angle prisms. In this case, the polarization rotating means are two elements, each located inside a groove-like space of the respective prism, and the beam directing means are represented by reflective surfaces of the beam splitter (the surfaces of the prisms). Alternatively, such a beam splitter may be made from a polarization rotating material, the polarization rotating means being thereby presented at two parts of the beam splitter at opposite sides, respectively, of the polarizing surface.
The beam splitter in another configuration may have three locally adjacent truncated corners forming three facets, the intermediate facet being that intercepting with the plane of the polarizing facet. In this case, the polarization rotating means are in the form of two plates located on the other two facets, respectively, and the beam directing means are represented by the rear reflective surfaces of the polarization rotating plates and by the intermediate facet of the beam splitter. Alternatively, in such a beam splitter with three facets, the polarization rotating means may be in the form of only one plate located on the intermediate facet, the rear reflective surface of this plate, and the reflective inner surfaces of other two facets serving as the beam directing means.
According to yet another aspect of the present invention, there is provided an all-optical switch device in the form of a polarizing beam splitter made of a polarization rotating material.
The all-optical switch device according to the invention may be used as a basic block in a multi-stage switch structure. In tis case, a required number of such basic devices are arranged in an array, and additional beam direct means are used for directing an output beam of one device to input a successive device.
Thus, according to yet another aspect of the present invention, there is provided a multi-stage all-optical switch structure comprising an array of at least first and second switch devices, each constructed as described above; and at least one beam directing element accommodated in an optical path of the output beam produced by the first switch device to direct said output beam onto a polarizing beam splitting surface of the second switch device.
The multi-stage switch structure composed of three switch devices, each constructed as described above, may be used for reducing crosstalk between output channels and/or for increasing the switch speed. In this case, the switch devices are arranged such that two output channels of the first device serves as two input channels of, respectively, the second and the third devices. One of the output channels of the second switch device and one of the output channels of the third switch device are blocked to prevent light output therethrough. By this, light signals collected at unblocked output channels of the second and third switch devices are characterized by reduced crosstalk. By increasing the number of switching stages in the switching structure, the crosstalk between output channels can be even more reduced. To increase a switching speed with the same switching structure composed of three switch devices, the polarization rotating means is operated to provide rotation of the polarization of the incident beam at an angle other than 90xc2x0.
The switch device according to the invention, can be used for multicast switching (generally, variable beam splitting). This is implemented by utilizing a CPR of the kind, where any desired difference in phase delay (from 0 to xcex/2) can be created between the two principle axes of the CPR material. This enables to obtain any desired partition between the output beam polarization components it the CPR output, and, consequently, any partition between the output channels of the switching device.
The above concept can be utilized for using the device according to the invention as a variable attenuator. This is implemented by blocking one of the output channels of the switch device, and, optionally, further combining several switch devices in series.
The present invention also provides for correcting errors that can be introduced by the splitting on the unpolarized input beam at the polarizing beam splitting surface. This is implemented by including an additional polarizing beam splitter cube in the beam directing means, and, optionally, also a polarization rotator (e.g., xcex/2 plate) in front of the additional polarizing beam splitter cube.
The present invention also provides for compensating the hysteresis phenomenon that can be observed with a CPR. This is implemented by appropriately controlling voltages applied to the CPR.
Additionally, the present invention provides for reducing switching differential voltage requirements. This is implemented by applying appropriate voltages to the CPR (depending on the CPR type) to cause phase delays of xcex/2 and xcex between the split beam components, rather than the phase delays of 0 and xcex/2.